Change play mechanism to better handle restart.
Change play mechanism to better handle restart.

 * The old method was nextTick/nextRow, which required that to start the
   song, the play cursor was place behind the actual start point. Now it
   uses processTick/processRow and processes the current row before
   advancing the cursor. This allows you to just set the cursor to the
   start and play.

pgregory/wetracker,pgregory/wetracker